This proposal aims to determine the role of neurotrophins and their receptors, the Trks, in modulating synapse structure and function in the developing central nervous system. Previous work by Gonzalez et al. (1999) demonstrated that at neuromuscular synapses, TrkB is localized to the postsynaptic membrane of muscle fibers and that TrkB mediated signaling modulates clustering of postsynaptic acetylcholine receptors. While the presynaptic role of TrkB in synaptic modulation in the CNS has been extensively studied, the role of postsynaptic TrkB-mediated signaling has been largely unexplored. To test the hypothesis that TrkB-mediated signaling plays a role in the maturation and maintenance of postsynaptic neurotransmitter receptor clusters at CNS synapses, influencing synapse structure and function, the localization of TrkA, B and C and their ligands in dissociated hippocampal neurons and histological sections and how their localization is modulated by activity will be determined. To determine how Trk-mediated signaling affects the structure and function of hippocampal synapses, recombinant adenoviruses and other methods will be used to over-express either dominant-negative, truncated Trks or full length and other mutant Trks in hippocampal neurons. Structural changes will be addressed using immunostaining and confocal microscopy, and functional changes will be assessed eletrophysiologically. The possibility that TrkB-mediated signaling modulates neurotransmitter receptor clustering and cluster maintenance is of interest and will be evaluated. Finally, the role of neuronal activity in regulating the trophic responsiveness of hippocampal neurons will be determined by examining the trafficking of Trk receptors and neurotrophins expressing fluorescent proteins tags that have been introduced into hippocampal neuron cultures. These experiments will aid in our understanding of the functional role of Trk-mediated signaling in synaptic formation and maintenance in the CNS and how trophic signaling and responsiveness are modulated by synaptic activity.